1. Field of the Invention
The present invention relates to telephone switching systems and, more particularly, to an office line selecting technique of a telephone switching system for selecting an optimal office line with respect to a specific extension.
2. Description of the Related Art
A private branch exchange accommodates many lines installed from a telephone office in its office line card and connects more extensions than the number of office lines thereto. Respective extensions have classes and their functions for providing service are determined according to the classes. In case of a very high class, the service of a special function may be provided. In selecting the office line, there is a method for selecting the office line using an office line number corresponding to the office line and a method for selecting the office line using an office line group number including the office line. The latter method selects the office line which is not used by sequentially judging whether the office line is being used. This method has a problem in that it is difficult to select an office line which has an optimal state with respect to the extension of a high class since the speech sensitivity and noise differ according to the office line.
The following patents each disclose features in common with the present invention but do not disclose the specifically recited combination of features of the present invention: U.S. Pat. No. 5,434,852 to La Porta et al., entitled Distributed Processing Architecture for Control Of Broadband And Narrowband Communications Networks, U.S. Pat. No. 5,208,803 to Conforti et al., entitled Circuit For Testing Digital Lines, U.S. Pat. No. 5,134,652 to Brown et al., entitled Communication Console Station With Priority Queuing, U.S. Pat. No. 4,958,369 to Tsuchida, entitled Key Telephone System Accommodating Both Digital And Analog Telephone Networks, and U.S. Pat. No. 3,660,611 to Knollman et al., entitled Program Controlled Key Telephone System For Automatic Selection Of A Prime Line.